13-MAC Address Table Management Operation

13-MAC Address Table Management Operation
Table of Contents
1 MAC Address Table Management············································································································1-1
Overview ·················································································································································1-1
Introduction to MAC Address Table ································································································1-1
Introduction to MAC Address Learning ···························································································1-2
Managing MAC Address Table ·······································································································1-4
Configuring MAC Address Table Management ······················································································1-5
MAC Address Table Management Configuration Task List ····························································1-5
Configuring a MAC Address Entry ··································································································1-5
Setting the MAC Address Aging Timer····························································································1-6
Setting the Maximum Number of MAC Addresses a Port Can Learn ·············································1-7
Enabling Destination MAC Address Triggered Update ···································································1-7
Assigning MAC Addresses for Ethernet Ports ················································································1-8
Displaying MAC Address Table Information ···························································································1-9
Configuration Example····························································································································1-9
Adding a Static MAC Address Entry Manually ················································································1-9
i
1
MAC Address Table Management
When configuring MAC address table management, go to these sections for information you are
interested in:
z
Overview
z
Configuring MAC Address Table Management
z
Displaying MAC Address Table Information
z
Configuration Example
z
This chapter describes the management of static, dynamic, and blackhole MAC address entries.
For information about the management of multicast MAC address entries, refer to the part related
to multicast protocol.
z
The function of destination MAC address triggered update was introduced. For more information,
refer to Enabling Destination MAC Address Triggered Update.
z
The function of assigning MAC addresses to Ethernet ports was introduced. For more information,
refer to Assigning MAC Addresses for Ethernet Ports.
Overview
Introduction to MAC Address Table
An Ethernet switch is mainly used to forward packets at the data link layer, that is, transmit the packets
to the corresponding ports according to the destination MAC address of the packets. To forward packets
quickly, a switch maintains a MAC address table, which is a Layer 2 address table recording the MAC
address-to-forwarding port association. Each entry in a MAC address table contains the following fields:
z
Destination MAC address
z
ID of the VLAN which a port belongs to
z
Forwarding egress port numbers on the local switch
When forwarding a packet, an Ethernet switch adopts one of the two forwarding methods based upon
the MAC address table entries.
z
Unicast forwarding: If the destination MAC address carried in the packet is included in a MAC
address table entry, the switch forwards the packet through the forwarding egress port in the entry.
z
Broadcast forwarding: If the destination MAC address carried in the packet is not included in the
MAC address table, the switch broadcasts the packet to all ports except the one receiving the
packet.
1-1
Introduction to MAC Address Learning
MAC address table entries can be updated and maintained through the following two ways:
z
Manual configuration
z
MAC address learning
Generally, the majority of MAC address entries are created and maintained through MAC address
learning. The following describes the MAC address learning process of a switch:
1)
As shown in Figure 1-1, User A and User B are both in VLAN 1. When User A communicates with
User B, the packet from User A needs to be transmitted to Ethernet 1/0/1. At this time, the switch
records the source MAC address of the packet, that is, the address MAC-A of User A to the MAC
address table of the switch, forming an entry shown in Figure 1-2.
Figure 1-1 MAC address learning diagram (1)
User B
User C
Eth1/0/4
Eth1/0/3
Eth1/0/1
User A
Figure 1-2 MAC address table entry of the switch (1)
1-2
2)
After learning the MAC address of User A, the switch starts to forward the packet. Because there is
no MAC address and port information of User B in the existing MAC address table, the switch
forwards the packet to all ports except Ethernet 1/0/1 to ensure that User B can receive the packet.
Figure 1-3 MAC address learning diagram (2)
3)
Because the switch broadcasts the packet, both User B and User C can receive the packet.
However, User C is not the destination device of the packet, and therefore does not process the
packet. Normally, User B will respond to User A, as shown in Figure 1-4. When the response
packet from User B is sent to Ethernet 1/0/4, the switch records the association between the MAC
address of User B and the corresponding port to the MAC address table of the switch.
Figure 1-4 MAC address learning diagram (3)
4)
At this time, the MAC address table of the switch includes two forwarding entries shown in Figure
1-5. When forwarding the response packet, the switch unicasts the packet instead of broadcasting
it to User A through Ethernet 1/0/1, because MAC-A is already in the MAC address table.
Figure 1-5 MAC address table entries of the switch (2)
1-3
5)
After this interaction, the switch directly unicasts the packets destined for User A and User B based
on the corresponding MAC address table entries.
z
Under some special circumstances, for example, User B is unreachable or User B receives the
packet but does not respond to it, the switch cannot learn the MAC address of User B. Hence, the
switch still broadcasts the packets destined for User B.
z
The switch learns only unicast addresses by using the MAC address learning mechanism but
directly drops any packet with a broadcast source MAC address.
Managing MAC Address Table
Aging of MAC address table
To fully utilize a MAC address table, which has a limited capacity, the switch uses an aging mechanism
for updating the table. That is, the switch starts an aging timer for an entry when dynamically creating
the entry. The switch removes the MAC address entry if no more packets with the MAC address
recorded in the entry are received within the aging time.
z
The MAC address aging timer only takes effect on dynamic MAC address entries.
z
With the destination MAC address triggered update function enabled, when a switch finds a packet
with a destination address matching one MAC address entry within the aging time, it updates the
entry and restarts the aging timer.
Entries in a MAC address table
Entries in a MAC address table fall into the following categories according to their characteristics and
configuration methods:
z
Static MAC address entry: Also known as permanent MAC address entry. This type of MAC
address entries are added/removed manually and can not age out by themselves. Using static
MAC address entries can reduce broadcast packets remarkably and are suitable for networks
where network devices seldom change.
z
Dynamic MAC address entry: This type of MAC address entries age out after the configured aging
time. They are generated by the MAC address learning mechanism or configured manually.
z
Blackhole MAC address entry: This type of MAC address entries are configured manually. A switch
discards the packets destined for or originated from the MAC addresses contained in blackhole
MAC address entries.
Table 1-1 lists the different types of MAC address entries and their characteristics.
1-4
Table 1-1 Characteristics of different types of MAC address entries
MAC address
entry
Configuration method
Aging time
Reserved or not at reboot
(if the configuration is
saved)
Static MAC
address entry
Manually configured
Unavailable
Yes
Dynamic MAC
address entry
Manually configured or
generated by MAC
address learning
mechanism
Available
No
Blackhole MAC
address entry
Manually configured
Unavailable
Yes
Configuring MAC Address Table Management
MAC Address Table Management Configuration Task List
Complete the following tasks to configure MAC address table management:
Task
Remarks
Configuring a MAC Address Entry
Required
Setting the MAC Address Aging Timer
Optional
Setting the Maximum Number of MAC Addresses a Port Can Learn
Optional
Enabling Destination MAC Address Triggered Update
Optional
Assigning MAC Addresses for Ethernet Ports
Optional
Configuring a MAC Address Entry
You can add, modify, or remove a MAC address entry, remove all MAC address entries concerning a
specific port, or remove specific type of MAC address entries (dynamic or static MAC address entries).
You can add a MAC address entry in either system view or Ethernet port view.
Adding a MAC address entry in system view
Follow these steps to add a MAC address entry in system view:
To do…
Use the command…
Remarks
Enter system view
system-view
—
Add a MAC address
entry
mac-address { static | dynamic | blackhole }
mac-address interface interface-type
interface-number vlan vlan-id
Required
1-5
z
When you add a MAC address entry, the port specified by the interface argument must belong to
the VLAN specified by the vlan argument in the command. Otherwise, the entry will not be added.
z
If the VLAN specified by the vlan argument is a dynamic VLAN, after a static MAC address is
added, it will become a static VLAN.
Adding a MAC address entry in Ethernet port view
Follow these steps to add a MAC address entry in Ethernet port view:
To do…
z
Use the command…
Remarks
Enter system view
system-view
—
Enter Ethernet port view
interface interface-type
interface-number
—
Add a MAC address entry
mac-address { static |
dynamic | blackhole }
mac-address vlan vlan-id
Required
When you add a MAC address entry, the current port must belong to the VLAN specified by the
vlan argument in the command. Otherwise, the entry will not be added.
z
If the VLAN specified by the vlan argument is a dynamic VLAN, after a static MAC address is
added, it will become a static VLAN.
Setting the MAC Address Aging Timer
Setting an appropriate MAC address aging timer is important for the switch to run efficiently.
z
If the aging timer is set too long, excessive invalid MAC address entries maintained by the switch
may fill up the MAC address table. This prevents the MAC address table from being updated with
network changes in time.
z
If the aging timer is set too short, the switch may remove valid MAC address entries. This
decreases the forwarding performance of the switch.
Follow these steps to set aging time of MAC address entries:
To do…
Use the command…
Remarks
Enter system view
system-view
—
Set the MAC address
aging timer
mac-address timer { aging age |
no-aging }
Required
1-6
The default is 300 seconds.
Normally, you are recommended to use the default aging timer, namely, 300 seconds. The no-aging
keyword specifies that MAC address entries never age out.
MAC address aging configuration applies to all ports, but only takes effect on dynamic MAC addresses,
which are either learnt or configured.
Setting the Maximum Number of MAC Addresses a Port Can Learn
The MAC address learning mechanism enables an Ethernet switch to acquire the MAC addresses of
the network devices on the segment connected to the ports of the switch. By searching the MAC
address table, the switch directly forwards the packets destined for these MAC addresses through the
hardware, improving the forwarding efficiency. A MAC address table too big in size may prolong the time
for searching MAC address entries, thus decreasing the forwarding performance of the switch.
By setting the maximum number of MAC addresses that can be learnt from individual ports, the
administrator can control the number of the MAC address entries the MAC address table can
dynamically maintain. When the number of the MAC address entries learnt from a port reaches the set
value, the port stops learning MAC addresses.
Follow these steps to set the maximum number of MAC addresses a port can learn:
To do…
Use the command…
Remarks
Enter system view
system-view
—
Enter Ethernet port view
interface interface-type
interface-number
—
Set the maximum number of
MAC addresses the port can
learn
mac-address max-mac-count
count
Required
By default, the number of the
MAC addresses a port can
learn is not limited.
If you have configured the maximum number of MAC addresses that a port can learn, you cannot
enable the MAC address authentication or port security functions on the port, and vice versa.
Enabling Destination MAC Address Triggered Update
By default, a switch updates its MAC address entries based on the source MAC addresses of packets.
However, this may cause the switch to perform unnecessary broadcasts in some applications. For
example, when a port aggregation group is used in an IRF fabric for communications, MAC address
entries of some ports in the aggregation group may not be updated in time, resulting in unnecessary
broadcasts.
1-7
The destination MAC address triggered update function solves the above problem by allowing the
switch to update its MAC address entries according to destination MAC addresses in addition to source
MAC addresses. This function improves the availability of the MAC address table.
Follow these steps to enable destination MAC address triggered update:
To do…
Use the command…
Remarks
Enter system view
system-view
—
Enable destination MAC
address triggered update
mac-address aging
destination-hit enable
Required
Disabled by default
Assigning MAC Addresses for Ethernet Ports
By default, no Ethernet port of an S3600 switch is configured with a MAC address. Therefore, when the
switch sends Layer 2 protocol packets, for example, BPDUs of STP, it uses the MAC address
predefined in the protocol as the source address to send the BPDUs. As switches in your network may
use the same source MAC address for Layer 2 protocol data units (PDUs), different ports on a switch
may learn the same MAC address, thus affecting the maintenance of the MAC address table.
To avoid the problem, you are allowed to assign MAC addresses to the Ethernet ports on an S3600
series switch. The idea is to assign a MAC address (called the start port MAC address) for the start
Ethernet port, that is, Ethernet 1/0/1, and each of the following ports uses the MAC address of the
preceding port plus 1 as its MAC address.
For example, if you configure 000f-e200-0001 as the start port MAC address, then port Ethernet 1/0/2
will take MAC address 000f-e200-0002, and so on.
Follow these steps to configure the start port MAC address:
To do…
Use the command…
Enter system view
system-view
Configure the start port MAC
address
port-mac start-mac-address
Remarks
—
Required
No start port MAC address is
configured by default.
The start port MAC address must be a valid unicast address.
With the port MAC address configuration, the switch uses the MAC address of a port as the source
MAC address when sending the following Layer 2 PDUs out of the port:
z
LACP
z
STP
z
NDP/NTDP
z
GVRP
z
DLDP
Port MAC address configuration does not affect service packet forwarding.
1-8
Displaying MAC Address Table Information
To do…
Use the command…
Display information about the MAC
address table
display mac-address
[ display-option ]
Display the aging time of the dynamic
MAC address entries in the MAC address
table
display mac-address aging-time
Display the configured start port MAC
address
display port-mac
Remarks
Available in any
view
Configuration Example
Adding a Static MAC Address Entry Manually
Network requirements
The server connects to the switch through Ethernet 1/0/2. To prevent the switch from broadcasting
packets destined for the server, it is required to add the MAC address of the server to the MAC address
table of the switch, which then forwards packets destined for the server through Ethernet 1/0/2.
z
The MAC address of the server is 000f-e20f-dc71.
z
Port Ethernet 1/0/2 belongs to VLAN 1.
Configuration procedure
# Enter system view.
<Sysname> system-view
[Sysname]
# Add a MAC address, with the VLAN, ports, and states specified.
[Sysname] mac-address static 000f-e20f-dc71 interface Ethernet 1/0/2 vlan 1
# Display information about the current MAC address table.
[Sysname] display mac-address interface Ethernet 1/0/2
MAC ADDR
VLAN ID
STATE
PORT INDEX
AGING TIME(s)
000f-e20f-dc71
1
Config static
Ethernet1/0/2
NOAGED
000f-e20f-a7d6
1
Learned
Ethernet1/0/2
AGING
000f-e20f-b1fb
1
Learned
Ethernet1/0/2
AGING
000f-e20f-f116
1
Learned
Ethernet1/0/2
AGING
---
4 mac address(es) found on port Ethernet1/0/2 ---
1-9
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Download PDF

advertisement